boost::mpl transform operation meta-function argument error

Question

In the Boost MPL web-documentation, it talks about passing meta-function classes as arguments to boost::mpl::transform. The meta-function argument, in this case, should be an operation of some sort to carry out on an mpl::ForwardSequence. However, when applying mpl::transform to an mpl::map using a simple meta-function class, I am getting template errors. (As these errors are quite extensive, I have included only what I believe to be the relevent bit. I am more than happy to post a more extensive error report if requested.)

Error:

/usr/include/boost/mpl/aux_/preprocessed/gcc/bind.hpp:207:21: error: no type named     ‘type’ in ‘struct boost::mpl::apply_wrap2<boost::mpl::push_front<mpl_::na, mpl_::na>, boost::mpl::map0<>, boost::mpl::pair<unsigned int, INT32U> >’
test_boost_mpl.cpp:106:1: error: ‘from_native_tmap’ was not declared in this scope

In my particular case my code looks like the following:

/* stl includes */
#include <cstdint>

/* boost includes */
#include <boost/type_traits.hpp>
#include <boost/type_traits/is_same.hpp>

#include <boost/mpl/at.hpp>
#include <boost/mpl/map.hpp>
#include <boost/mpl/empty.hpp>

#include <boost/mpl/assert.hpp>
#include <boost/mpl/transform.hpp>

struct Boolean {
  enum { tag_value = 0x83 };
};

struct INT32U {
  enum { tag_value = 0x84 };
};

typedef mpl::map
< 
  mpl::pair<Boolean, bool>,
  mpl::pair<INT32U, std::uint32_t>
> to_native_tmap;

struct swap_f {
  template<typename PAIR>
  struct apply {
    typedef typename mpl::pair<typename PAIR::second, typename PAIR::first> type;
  };
};

typedef mpl::transform<to_native_tmap, swap_f>::type from_native_tmap;

BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, bool>::type, Boolean> ));
BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, std::uint32_t>::type, INT32U> ));

int main(void) { return 0; }

My intent is have a mapping to native c++ types in to_native_tmap and then the reverse mapping in from_native_tmap.

This code fails on either BOOST_MPL_ASSERT() or if I attempt to instantiate the to_native_tmap mpl::map type.

A big 'Thank You!' up front to anyone willing to help.

Answer 1

/* stl includes */
#include <cstdint>

/* boost includes */
#include <boost/type_traits.hpp>
#include <boost/type_traits/is_same.hpp>

#include <boost/mpl/at.hpp>
#include <boost/mpl/map.hpp>
#include <boost/mpl/empty.hpp>

#include <boost/mpl/assert.hpp>
#include <boost/mpl/transform.hpp>

#include <boost/mpl/inserter.hpp> //ADDED
#include <boost/mpl/insert.hpp> //ADDED

namespace mpl= boost::mpl; //ADDED
using boost::is_same; //ADDED

struct Boolean {
  enum { tag_value = 0x83 };
};

struct INT32U {
  enum { tag_value = 0x84 };
};

typedef mpl::map
< 
  mpl::pair<Boolean, bool>,
  mpl::pair<INT32U, std::uint32_t>
> to_native_tmap;

struct swap_f {
  template<typename PAIR>
  struct apply {
    typedef typename mpl::pair<typename PAIR::second, typename PAIR::first> type;
  };
};

//The default inserter used by mpl::transform requires that the container have a push_front algorithm
//This isn't the case for mpl::map (as the assertion in g++4.8.0 reveals: REQUESTED_PUSH_FRONT_SPECIALIZATION_FOR_SEQUENCE_DOES_NOT_EXIST)
//In this case you need to add a custom inserter
typedef mpl::inserter<mpl::map0<>, mpl::insert<mpl::_1,mpl::_2> > map_inserter; //ADDED
typedef mpl::transform<to_native_tmap, swap_f, map_inserter >::type from_native_tmap; //CHANGED

BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, bool>::type, Boolean> ));
BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, std::uint32_t>::type, INT32U> ));

int main(void) { return 0; }